The difficulty is going to reset in a little over a day. It's going to be a 50% increase, which means you will make 2/3 what you make today, starting Friday at 6:00 AM or so.

Today, Tomorrow: 0.79/dayThe 8-10 days after that: 0.521/day

So about 15.64 BTC per month -- but that assumes there are no more difficulty increases for the next 30 days. Which is not likely! It hasn't happened yet (in the post-May-2011 era we're in; namely, since most people found out about Bitcoin)

Then you have to place your bet on how much a BTC will be worth. $15? $10? $30?

One thing to keep in mind -- users of BTC don't feel any need to pay more for a BTC, just because difficulty went up. That's a common fallacy. If anything, higher price will lead to higher difficulty, as greedy miners buy more equipment and make the difficulty go up. That actually makes sense. But the other way around? No.

The same number of BTC will be produced in a week (6/hour, creeping up to 8 or 10/hour after 8 days) whether the "bounty" is divided up by 1000 miners, 1500 miners, or 2000 miners. Adding more miners to the system makes BTC harder to produce, but the same number are produced when all is said and done.

Yes, we happened to have a run-up in price as difficulty rose twice in late May/early June. But that was a coincidence; a product of a manic bubble which spectacularly burst at $32. I haven't seen any $30 BTC for a while. Wish I had! I'm a miner myself, so I'd love to see $30 BTC. But I'd also like to sprout wings and fly. Neither are likely, unfortunately for me.

If you want to be conservative in your expectations -- or super-optimistic -- that's up to you. All you can do is run the numbers and decide if it's for you or not.